1. Field of the Invention
The present invention relates to a voltage-controlled oscillator operating in a microwave or millimeter wave region and, more particularly, to a voltage-controlled oscillator capable of easily correcting variation in oscillation frequency with temperature and shifting the range of voltage applicable across a variable-capacitance element to a region on the high-voltage side.
2. Background Art
FIG. 4 is a circuit diagram showing an example of a conventional voltage-controlled oscillator. The voltage-controlled oscillator shown in FIG. 4 has a voltage-controlled oscillation section 21 which controls the oscillation frequency by a voltage applied to a variable-capacitance element 6, and a frequency control bias circuit 7 which applies a frequency control bias to one end of the variable-capacitance element 6.
The voltage-controlled oscillation section 21 has a bipolar transistor 1, an output matching circuit 2 connected to the collector of the bipolar transistor 1, a series feedback inductor 3 connected to the emitter of the bipolar transistor 1, a phase adjustment line 4 having its one end connected to the base of the bipolar transistor 1 and satisfying an oscillation start phase condition, an inductor 5 and the variable-capacitance element 6. The inductor 5 and the variable-capacitance element 6 are connected between the other end of the phase adjustment line 4 and a grounding point and constitute an LC series resonance circuit. The frequency control bias circuit 7 is a resistance-feed-type bias circuit which applies to one end of the variable-capacitance element 6 a frequency control bias externally supplied through a frequency control bias terminal B.
FIG. 5 is a diagram schematically showing oscillation frequency characteristics of the conventional voltage-controlled oscillator. Conventionally, the voltage applied to the variable-capacitance element 6 is controlled according to temperature in order to correct variation in oscillation frequency with temperature. For example, in a case where the voltage-controlled oscillator is used by controlling the voltage to point A in FIG. 5 at ordinary temperature, the voltage is controlled to point B when the temperature is high, and is controlled to point C when the temperature is low, thus obtaining the same frequency at different temperatures.
The voltage-controlled oscillator shown in FIG. 4, however, needs to perform frequency control and temperature compensation through one terminal and therefore has a problem that it is necessary to apply a frequency control bias in a complicated way according to temperature.
FIG. 6 shows a conventional voltage-controlled oscillator devised to solve this problem (see, for example, Japanese Patent Laid-Open No. 62-21304). This voltage-controlled oscillator has, in addition to the components of the voltage-controlled oscillator shown in FIG. 4, a temperature compensation bias circuit 10 which applies a temperature compensation bias to the other end of the variable-capacitance element 6 and a temperature compensation bias generation circuit 22 which generates a temperature compensation bias and supplies the generated bias to the temperature compensation bias circuit 10. The voltage-controlled oscillation section 21 further has a direct current blocking capacitor 9 connected between the variable-capacitance element 6 and a grounding point.
The temperature compensation bias generation circuit 22 is a resistance-feed-type bias circuit. A frequency control bias supplied from the temperature compensation bias generation circuit 22 via point X is applied to the other end of the variable-capacitance element 6. The temperature compensation bias generation circuit 22 has a bipolar transistor 11 having its collector connected to the temperature compensation bias circuit 10, a resistor 12 having its one end connected to the collector of the bipolar transistor 11, a resistor 13 having its one end connected to the base of the bipolar transistor 11, a base bias application terminal 14 connected to the other end of the resistor 13, a collector bias application terminal 15 connected to the other end of the resistor 12, and a resistor 16 having its one end connected to the emitter of the bipolar transistor 11 and having the other end grounded.
While in the voltage-controlled oscillator shown in FIG. 4 the oscillation frequency becomes lower with the increase in temperature, the provision of the temperature compensation bias circuit 10 and the temperature compensation bias generation circuit 22 in the voltage-controlled oscillator shown in FIG. 6 enables the oscillator to output of a signal at a fixed frequency even when the temperature rises. The reason for this effect is as described below. With the increase in temperature, the collector current of the bipolar transistor 11 increases to increase the voltage drop across the resistor 12 and reduce the voltage at point X. The voltage across the variable-capacitance element 6 is thereby increased to increase the oscillation frequency, thus correcting the reduction in oscillation frequency due to an increase in temperature. As a result, there is no need to apply the frequency control bias in a complicated way according to temperature.
In the voltage-controlled oscillator shown in FIG. 6, the voltage at point X decreases with the increase in temperature, as shown in FIG. 7, but converges to a certain point indicated by Y in FIG. 7. An additional offset voltage corresponding to Y is thus added to the voltage used primarily for temperature compensation, so that the range of voltage applicable across the variable-capacitance element 6 is shifted to a region on the low-voltage side.
A reverse-biased diode is ordinarily used as a variable-capacitance element. As the above-mentioned value Y is increased, the voltage across the diode becomes closer to a forward bias. Therefore, the diode can be only used before a point at which the voltage across the diode becomes equal to the on voltage, as shown in FIG. 8. There is a possibility of failure to make sufficient temperature compensation. Moreover, when the voltage across the diode becomes closer to a forward bias, low-frequency noise from an external power supply and low-frequency noise generated in the diode are up-converted to the oscillation frequency band by the non-linear operation of the diode, resulting in an increase in phase noise.
For example, in the case of a voltage-controlled oscillator for a vehicle radar, the output frequency of the voltage-controlled oscillator is generally ⅛ or more of a signal frequency of 76 to 77 GHz of the vehicle radar. In the case of a voltage-controlled oscillator operating at such a high frequency, the influence of assembly accuracy on the characteristics is large and, therefore, it is desirable, from the viewpoint of productivity, to design the voltage-controlled oscillator in a monolithic (MMIC) form. However, a resonator of a high Q factor such as a dielectric resonator cannot be made on a semiconductor substrate. Therefore, a resonator provided in MMIC form has larger phase noise in comparison with those using a dielectric resonator. It is difficult to obtain a resistor in MMIC form having a sufficient margin with respect to the phase noise performance required by the vehicle radar system. For this reason, the above-described increase in phase noise in the vicinity of a forward bias has been serious.